1. Field of the Invention
The present invention relates to a centrifugal supercharger, and more particularly to a bearing mechanism for a centrifugal supercharger.
2. Description of the Prior Art
A conventional bearing mechanism for a centrifugal supercharger is disclosed, for example, in Japanese patent application laid-open publication No.56(1981)-138423. As shown in FIG. 13, this conventional mechanism includes two radial bearings 92 which rotatably support a shaft 93 on a bearing housing 91 of a centrifugal supercharger and an intermediate sleeve 94 which is disposed between the radial bearings 92. In the bearing housing 91, an oil supply hole 95 is formed and oil is supplied from a oil supply source (not shown) to the radial bearings 92 through the oil supply passage 95.
Accordingly, since the oil is supplied to clearances which are formed between the radial bearings 92 and the shaft 93 rotating with high speed and an oil film is formed in the clearances, respectively, the shaft 93 rotates while floating on the radial bearings 92. In this situation, the radial bearings 92 are dragged by the rotation of the shaft 93 through the oil films and therefore are rotated with a rotational speed of 20-30% of the rotational speed of the shaft 93. An oil film is formed in a clearance between the bearing housing 91 and each of the radial bearings 92, too.
Thereby, since the relative velocity of the sliding surfaces of the radial bearings 92 and a sliding surface of the shaft 93 is decreased and furthermore a radial vibration of the shaft 93 is inhibited by a damping effect of the oil film, the radial bearings 92 carries the rotation of the shaft 93 stably without binding due to a frictional heat under the condition which the shaft 93 is rotated with high speed.
As mentioned above, in the above conventional bearing mechanism, the oil film which is formed on the inner and outer circumferential surfaces of the radial bearings 92, respectively functions as a damper for damping the vibration of the shaft 93. In practice, however, it is not possible to sufficiently obtain the effect for damping the vibration of the shaft 93 by these oil films. Therefore, the rotational vibration of the shaft 93 is transmitted to the bearing housing 91 through the radial bearings 93 or the rotational vibration of the radial bearing 92 is transmitted to the bearing housing 91. As a result, a noise is generated and this noise becomes a problem in a centrifugal supercharger for a passenger car when silence is desired.